Watchdog says any deployment will have to receive security clearance, a day after UN team comes under fire in Douma.

Inspectors from the global chemical weapons watchdog have not been able to enter Syria's Douma to examine the site of a suspected gas attack, the organisation said, dismissing an earlier report circulated by Syrian state-run media.

The Organisation for Prohibition of Chemical Weapons (OPCW) said on Wednesday that United Nations security officials entered the town on Tuesday to survey the area where its fact-finding mission was supposed to probe allegations of a chemical weapons attack carried out by government forces on April 7.

But the UN Department of Safety and Security (UNDSS) came under small arms fire, and an explosive device was detonated nearby during their stop at the second of the two sites that they visited, the OPCW said in a statement.

At the first site, the officials had to withdraw because a large crowd gathered there, raising security concerns. There were no injuries, and the UN team returned to Syria's capital, Damascus.

"The UNDSS will continue to work with the Syrian National Authority, the local Councils in Douma, and the Russian Military Police to review the security situation. At present, we do not know when the [Fact-Finding Mission] team can be deployed to Douma," the statement said.

Such deployment would only be considered following approval by the UN security team, and provided the OPCW team can have unhindered access to the sites, it added.

Syrian and Russian forces gained control over Douma on Saturday when rebels withdrew from the town, hours after the end of the Western countries' attacks.

On Tuesday, SANA falsely reported that the OPCW's fact-finding team, which arrived in Damascus on Saturday, entered Douma.

Punitive strikes

The OPCW team is tasked with investigating the reported toxic gas attack, but cannot apportion blame.

According to rescuers and medics, dozens of people were killed in the attack, which sparked retaliatory joint missile strikes by the United States, France and Britain on Syrian military installations.

Damascus and Moscow have denied that a chemical attack took place.

During an emergency meeting on Monday at the OPCW's headquarters in The Hague, Western diplomats accused the Syrian government and its Russian ally of blocking the team.

Russia denied the claims, saying parts of Douma still needed to be de-mined and said the watchdog's inspectors would enter on Wednesday.

Yet, France and the US appeared to question the purpose of such a mission, warning that any incriminating evidence had likely been removed by now.

Ishak Majali, a former OPCW inspector, said it is unlikely the inspectors will find evidence at the site after such delay.

"It has been a lot of time since the attack took place - we are talking about 11 days," he said.

"So, if you are in control of a site with chemicals for such a long time, it's very easy actually to tamper with the place and to change the facts on the ground," he added.

"You can actually do what we call it in the military business as the decontamination process, which is to remove all the evidence on the ground by using other chemicals to neutralise the chemicals on the ground. Also, you can tamper with the munition itself ... to prepare some witnesses or to prepare some medical reports."